It has been recognized that certain types of electrical devices can have peak current requirements which exceed average current requirements by several orders of magnitude. One form of such device is a strobe unit of the type used to indicate an emergency condition in an alarm system, such as a fire alarm.
FIG. 1 illustrates a known form of strobe unit 10. The strobe unit 10 includes control circuitry 12, which among other functions, manages charging, via circuitry 14, a relatively large electrolytic capacitor 16. Capacitor 16 is used to provide electrical energy to a gas discharge tube 18 which can be selectively triggered. When triggered, the electrical energy previously stored on capacitor 16 is coupled to tube 18 to provide a high intensity visual output indicative of the alarm condition.
When power is initially applied to such units, for example, via lines 22 as would be understood by those of skill in the art, an initial current surge which might have an amplitude as great as 10 amps can be drawn by the device 10. FIG. 2 is an exemplary graph illustrating an initial maximum current surge Io. The initial surge current is primarily due to a need to charge one or more electrolytic capacitors, such as capacitor 16, in strobe unit 10. After the initial current surge has subsided, the device 10 draws a substantially lower current, Irms, which typically depends on the candela output of the device 10. Such currents can fall in the range of less than 50 mA to more than 500 mA.
By design, strobe units, such as the unit 10 flash their respective output device 18 once a second as illustrated in FIG. 2. Each flash produces a substantial repeat current surge, Irep, which, while less than the initial current surge Io, can still be orders of magnitude above the intermittent current demands Irms. Those of skill will understand that the peak current values illustrated in FIG. 2 are exemplary only and could vary between the strobe units depending on their exact design. Nevertheless, in each instance, such units exhibit an initial peak inrush current followed by repetitive, though lower, repeat inrush currents.
It has also been recognized that there is a benefit to incorporating flexibility into strobe units, such as the strobe unit 10 by providing a candela select capability 28 (illustrated in phantom in FIG. 1). This capability can be implemented with jumpers or switches located on the unit 10. This capability enables an installer to select one of a plurality of candela outputs, at installation, and have the benefits of a common product.
The presence of the initial peak current draw and repetitive peak current draws, as illustrated in FIG. 2 is undesirable. It has resulted in the use of current limiting circuitry 30, illustrated in phantom in FIG. 1, in strobe units, such as the unit 10. One such configuration has been disclosed in U.S. patent application Ser. No. 10/040,968 filed Jan. 7, 2002 and entitled “Processor Based Strobe with Feedback”, now U.S. Pat. No. 6,661,337. The U.S. Pat. No. 6,661,337 patent is assigned to the Assignee hereof and incorporated by reference herein. While the current limiting circuitry of the subject patent is effective for its intended purpose, it is primarily analog in nature and requires the inclusion of capacitors in the respective strobe units. Capacitors, of course, add both complexity and cost to such products.
There thus is a continuing need for current limiting circuitry which could be incorporated into strobe units, such as the exemplary strobe unit 10, to limit not only the initial peak in rush current but subsequent repetitive peak current values. Preferably, such circuitry could be implemented so as to minimize any additional costs without unduly requiring additional capacitors for the purpose of smoothing, and or limiting the initial peak current as well as repetitive peak current draws. Such circuitry might be useful in connection with other types of devices which draw substantial initial in-rush currents and/or subsequent repetitive peak currents.